Yea natural 1s always fail. This case of thinking about modifying the dice and not the save is normally not needed, as this is only relevant for a 1+ base save.
e.g rend 2 attack vs basti. Basti player rolls a 3. the roll is modified with -2 and becomes a 1. Basti player refers to warscroll which says 1+, the wound is saved. Even with 1000 rend the dice can not go lower than 1. However if the basti player rolls a natual 1, the save still fails.
As soon as the save is a 2+, the save roll will fail if the modified roll becomes a 1, so now it is entirely different, as the above -2 rend attack would make the roll of 3 into 1 modified 1, failing the save. With a 2+ save even a rend 1 attack will cause a roll of 2 to be modified to a 1 and failing, so after the basty takes the first 2 wounds, it will already bracket there and be fully vulnerable to rend as "normal".
No other model has a 1+ base save, which is why this has not come up before.